1. Field Of The Invention
This invention relates to a matrix switch device which has output lines extending along the row dimension and drive lines extending along the columnar dimension and in which a number of switch elements are arranged via the output and drive lines and, in particular, to an N-key rollover circuit in a matrix switch or the like which prevents "phantom key input" as a result of simultaneous depression of several keys.
2. Description Of The Related Art
A key board switch device is known in which, as in a membrane switch, a number of switch elements are arranged along the row and columnar dimensions and in which characters, figures, and other symbols are input by depressing the keys connected to the relevant switch elements.
FIG. 3 is a partial schematic diagram illustrating the circuit of a keyboard switch of this type. The drawing shows only two rows and two columns of the matrix switch device. Numerals 1 and 2 indicate output lines. Symbols A and B indicate drive lines. Numerals 1A, 1B, 2A and 2B indicate switch elements, and numerals O.sub.1 and O.sub.2 indicate output terminals (outputs 1 and 2).
Referring to the drawing, the switch elements 1A, 1B, 2A and 2B are turned ON by depressing keys associated therewith (not shown). Assuming that the drive line A has been selected by applying a scanning pulse SP thereto, turning ON the switch element 1A results in the formation of a closed circuit: drive line A.fwdarw.switch element 1A.fwdarw.output line 1.fwdarw.output terminal O.sub.1, and a current I.sub.1 flows to the output terminal O.sub.1, with the result that a signal ("1" output) indicating the ON state of the switch element 1A is obtained at the output 1. At this time, no current flows to the other output terminals than the output terminal O.sub.1, so that the other output terminals indicate the OFF ("O") state.
In this construction, if, when the drive line A has been selected, other switch elements, 1B and 2B, are turned ON, in addition to the switch element 1A as a result of simultaneous depression of several keys, another closed circuit is formed: drive line A.fwdarw.switch element 1A.fwdarw.output line 1.fwdarw.switch element 1B.fwdarw.drive line B.fwdarw.switch element 2B.fwdarw.output line 2.fwdarw.output terminal O.sub.2, in addition to the above-mentioned closed circuit: drive line A.fwdarw.switch element 1A.fwdarw.output line 1.fwdarw.output terminal O.sub.1, and a current I.sub.2 flows to the output terminal O.sub.2, with the result that a signal indicating that the switch element 2A is ON is obtained at the output 2, though that is not the case.
That is, in this case, the outputs 1 and 2 are both made "1". In other words, the output 2, which ought to be "0", is made "1", resulting in a so-called "phantom key" state, which leads to malfunction.
A conventional N-key rollover circuit which overcomes the above problem will now be described with reference to FIG. 4. In the drawing, numerals 3 and 4 indicate comparators; numeral 5, a grounding means; numerals 6 and 7, changeover switches; symbol Vref, a threshold voltage source for the comparators 3 and 4; and symbol GND, the ground potential. The components corresponding to those of FIG. 3 are indicated by the same reference numerals.
Referring to FIG. 4, resistors R1A, RIB, R2A and R2B are connected in series to the switch elements 1A, 1B, 2A and 2B, respectively, which switch elements are provided at the crosspoints of the output lines 1 and 2 and the drive lines A and B which constitute the rows and columns of the matrix. These series resistors have the same resistance value.
The grounding means 5 is composed of changeover switches 6 and 7 for reducing the potential of the drive lines other than the drive line to which the scanning pulse SP is applied (the drive line A in the drawing), that is, the potential of the drive line B (shown in this case), to the ground potential GND.
Assuming that the drive line A is selected by applying the scanning pulse SP, and that only the switch element 1A is turned ON, the current I.sub.1 flows through the route: drive line A.fwdarw.switch element 1A.fwdarw.series resistor R1A.fwdarw.output terminal O.sub.1, and the voltage level thereof is compared with the threshold value V.sub.ref by the comparator 3, whereby the output 1 is recognized as ON ("1").
If, simultaneously with turning ON the switch element 1A, the switch element 1B in the same row (the output line 1) is also turned ON, the current 1.sub.2 flows, in addition to the above current I.sub.1, through the route: drive line A.fwdarw.switch element 1A.fwdarw.series resistor R1A.fwdarw.output line 1.fwdarw.series resistor R1B.fwdarw.switch element 1B.fwdarw.drive line B.fwdarw.ground, since the drive line B has been grounded by the changeover switch 7 of the grounding means. Accordingly, the signal level of the output 1 is reduced through division of the crest value Vin of the scanning pulse SP by the series resistors R1A and RIB.
Thus, even if, when the drive line A has been selected by applying the scanning pulse SP, switch elements other than the switch element 1A, connected to the output line 1, are erroneously turned ON, though only the switch element 1A connected to the output line 1 ought to have been turned ON, the comparator 3 can detect the voltage at the output line 1.
Further, as shown in FIG. 4, if the switch element 2B is also turned ON, no voltage is generated in the output line 2 since the drive line B is grounded, so that the output 2 of the output terminal O.sub.2 is not "1". Thus, it is possible to prevent erroneous recognition due to simultaneous depression of several keys.
The threshold value V.sub.ref set in the comparators 3 and 4 is set to the minimum level of the same output line at the time of simultaneous key depression of the maximum number of columns.
In the N-key rollover circuit shown in FIG. 4, the output of the comparator is "1" only when the voltage level of the output line is higher than the threshold value V.sub.ref. As described above, this threshold value V.sub.ref should be set lower than the voltage level of the same output line at the time of simultaneous key depression of the maximum number of columns. However, setting this value too low is undesirable since that would restrict the number of switch elements (the number of key switches) in the same output line.
As regards the output line 1 of FIG. 4, the circuit configuration thereof is as shown in FIG. 5(a), and the equivalent circuit thereof is as shown in FIG. 5(b). Here, apart from the switch element 1A, N switch elements 1B, 1C, . . . , 1N are connected to the output line 1. Thus, series resistors R1A, RiB, R1C, . . . , R1N, are connected in series to these switch elements 1A through 1N, respectively.
Assuming that all the switch elements 1B through 1N are ON, the series resistors R1B through R1N are in a relationship of parallel connection. Assuming that the combined resistance value of them is RN, the equivalent circuit of FIG. 5(a) is as shown in FIG. 5(b), and the voltage v.sub.1 generated in the combined resistance RN is compared with the threshold value V.sub.ref by the comparator 3.
Suppose the number of switch elements connected to one output line is N; R1A=R1B= . . . =R1N=10 k.OMEGA.; and V.sub.ref =0.5 V. If, in this condition, the switch elements 1B, 1C, . . . , 1N are all ON, and the switch element 1A is turned ON to supply a scanning pulse SP of +5 V to the drive line, the combined resistance RN of the series resistors R1A, RB, . . . , R1N at this time is 10/Nk.OMEGA., so that the voltage V.sub.1 supplied to the comparator 3 is: 5.times.RN/(RN+R1A)=5/(1+N). When the threshold value V.sub.ref is set, for example, to 0.5 V, the condition which makes the voltage V.sub.1 equal to or larger than 0.5 V is as follows: EQU 0.5&lt;5/(1+N) Therefore, N&lt;9
That is, when the number of switch elements connected to one output line exceeds 9, the comparator 3 will make an erroneous judgment if ten or more switch elements are simultaneously turned ON.
Thus, the number of key switches used is limited. This might be avoided by further reducing the threshold value V.sub.ref. However, reducing the value to a level below 0.5 V will make it impossible to discriminate the value from noises or the like.
Further, there is a tendency to reduce the voltage of the scanning pulse SP (operating voltage) to approximately 3 V in order to attain a reduction in power consumption, etc. This again limits the number of key switches.
In addition, there is a dispersion in the resistance values of the series resistors R1A, . . . , and the switch elements 1A, . . . . Thus, to augment the number of key switches, a resistance printing of high precision is required, with the result that the key switches become expensive.